Liquid metal which is cast using intermediate pouring vessels, herein referred to as tundishes, is not always afforded sufficient residence time to allow non-metallic inclusions to become separated from the metal, or to allow entrained gases to escape and thus to produce a cast metal of desired purity and soundness. Evidently, residence time can be increased by providing tundishes of large capacity. Nevertheless, even in tundishes of particularly large capacity, for example, twenty thousand kilograms, the problem of insufficient residence time may be encountered. This is a result of large volumes of liquid metal occupying "dead spaces" in the tundish wherein the metal does not circulate and which reduce the effective capacity of the tundish.
In order to improve the circulation of liquid metal into "dead spaces", and thereby increase the residence time of the liquid metal, tundishes have been developed in which the metal flow is diverted using barriers. These barriers are usually but not necessarily substantially rectangular in cross-section and extend between the tundish walls in a direction which is transverse to the prevailing flow of metal, between the metal entry and exit points. One type of barrier, which is commonly called a weir, is located at the upper part of the tundish walls and prevents liquid metal from flowing continuously across the surface of the metal bath while permitting the liquid metal to flow underneath the weir. Another type of barrier, which is commonly called a dam, is located between the tundish walls at the lower part thereof and protrudes from the tundish floor. The flow of liquid metal in a tundish which is equipped with a dam is directed to move over the surface of the dam. Conveniently, dams and weirs may be used together in order to produce a desired liquid metal flow pattern.
Other developements to improve liquid metal circulation include the incorporation of porous plugs in the tundish floor. In use, the plugs are adapted to encourage metal circulation by introducing a flow of inert gas into the molten metal bath. Problems associated with this method include the reliability of the plugs to retain the molten metal, and the effectiveness of the relatively small surface areas of the plugs.
Still further developments include the incorporation of a gas distribution apparatus into a weir which in use, distributes inert gas bubbles through the liquid metal from the undersurface of the weir. The gas bubbles are emitted under pressure and tend to rise in the immediate vicinity of the weir. Their distribution is thus very limited, and the circulation of liquid metal is confined to the areas adjacent to the weir's lateral surfaces. As a result, the main flow of metal entering the tundish sill bypasses large areas of the tundish, and in effect, remains characterized by a short residence time.
It is the object of this invention to improve the quality of liquid metal before casting, by increasing the removal of non-metallic inclusions, and entrained gases, at least to a small extent, and also by providing a pouring stream of relatively uniform temperature and composition through the promotion of metal circulation in tundishes used for holding liquid metal during casting.